The technical field of this disclosure is Micro Electro Mechanical Systems (MEMS), particularly, MEMS scanning micromirrors.
MEMS scanning micromirrors have been developed for the display of visual information. The MEMS scanning micromirror oscillates in one or two dimensions and a laser or other light beam reflects from the mirror surface. Varying the angle and timing of the beam incident on the mirror surface generates a visual image on a screen or other surface, such as a two dimensional display matrix. Different numbers of MEMS scanning micromirrors and lasers are used to produce images of different detail and colors. Exemplary uses for the MEMS scanning micromirrors are head up displays for automotive applications, wearable displays, projection displays, mobile phone and hand-held displays, and barcode scanners.
The present generation of MEMS scanning micromirrors includes a mirror plate attached to a frame by two collinear torsion beams, which create a scanning axis about which the mirror plate rotates. The torsion beams both support the mirror plate and provide the required torsional stiffness during rotation. The torsion beams are the only point of attachment between the mirror plate and the frame, and determine the resonant frequency of the MEMS scanning micromirror. The MEMS scanning micromirror also includes a driver to magnetically or electrically apply a torque to the mirror plate about the scanning axis without physical contact with the mirror plate. The driver typically drives the mirror plate at the resonant frequency. MEMS scanning micromirrors are made from single crystal silicon or polysilicon material using photolithography.
Problems with image quality in the present generation of MEMS scanning micromirrors occur due to undesired motion of the mirror and mirror deformation. High image resolution is desirable for better image quality and larger displays. Because the torsion beams are the only point of attachment between the mirror plate and the frame, the torsion beams define the suspension stiffness in all directions, not only rotational stiffness about the scanning axis. The torsion beam geometry, with the mirror mass and the mass inertia moment for the corresponding rotation, influence the higher order resonant frequencies of the system. The most important higher order resonant modes are out-of-plane rocking mode, vertical translation mode, in-plane rotation mode, and horizontal translation mode. Out-of-plane rocking and the vertical translation resonant modes in the present generation of MEMS scanning micromirrors greatly reduce the image quality of the scanner display.
Micromirror dynamic deformation also reduces the image quality of the scanner display. Deformation should not be higher than ±λ/10, where λ is the shortest laser wavelength used in the scanning application. Image resolution is proportional to the product of scanning angle and micromirror diameter. Higher image resolution requires larger scanning angles, which increase the stress in the torsion beam, and higher scanning frequencies, which increase micromirror dynamic deformation. Because the torsion beams are the only point of attachment between the mirror plate and the frame, the torsion beams deform the mirror plate and the micromirror.
It would be desirable to have a MEMS scanning micromirror that would overcome the above disadvantages.